The present invention relates to lithography masks, particularly to transmission and attenuating phase-shift masks, and more particularly to a defect tolerant transmission lithography mask which has reduced optical absorption and reduced thermal loading.
Over the years various types of lithography masks have been developed in efforts to improve the lithographic technology. The following exemplifies prior masks.
Lithographic printing with 36 nm radiation using transmission masks of the open stencil type has been demonstrated by V. D. W. Berreman et al., "Soft-X-Ray Projection Lithography, Printing of 0.2 .mu.m Features Using 20:1 Reduction," Optics Letters 15, pp. 529-531 (1990). The transmission mask was formed using a combination of electron-beam lithography and reactive-ion etching to open patterned holes in a 1 mm.times.1 mm.times.700 .mu.m thick silicon (Si) membrane. A film of 30 .mu.m of gold (Au) was deposited on both sides of the membrane to increase the mask contrast.
A reflection type lithography mask usable to print a pattern upon a semiconductor wafer by the use of soft x-rays is disclosed in U.S. Pat. No. 5,052,033 issued Sep. 24, 1991, to Ikeda et al. In this patent all embodiments utilize reflective and nonreflective regions of the mask to provide a high contrast pattern. The reflective region consists of a multilayer film formed on a substrate by layering different materials having different refractive indices. Two approaches are identified therein to fabricate the patterned mask: (1) a nonreflective portion is formed on the reflecting surface, and (2) a pattern comprising the multilayer structure may be formed upon a nonreflective substrate.
Potential sources of defects in the open-stencil mask design include nonuniform membrane thickness, sidewall slope errors, and nonuniformity of the overcoat that serves to adjust the optical contrast. While repair of the overcoat is straightforward, repair of membrane thickness errors and sidewall slope errors presents a daunting technical challenge.
Reflection mask designs employing multilayer coatings require that the multilayer reflective regions of the mask are free of defects--there is no practical method for repairing defective regions of the multilayer coating. Defects in the multilayer coated regions of the sample can arise from several sources, including both structural flaws in the coating, and substrate imperfections that are replicated in the multilayer coating process.
Finally, the open-stencil and reflection mask designs absorb radiation that is not transmitted through the open region or reflected by the multilayer coating. During lithographic exposure, the mask is subject to a spatially varying thermal load produced by pattern-dependent optical absorption; this results in a spatially varying distortion of the mask pattern.
The present invention overcomes the above-identified problems by providing a defect tolerant transmission lithography mask, which provides several advantages over conventional lithography mask designs, which include being insensitive to defects in the membrane and reflective coating, is independent of the thermal load during exposure, and for extreme ultraviolet (EUV) it circumvents phase defect problems. The transmission lithography mask of the present invention utilizes a transparent substrate or a partially transparent membrane as the active region of the mask, on which is deposited a reflective coating on only the side facing the illumination system; the coating is patterned to form transmissive regions. Since the mask "discards," rather than absorbs, unwanted radiation, it has reduced optical absorption and reduced thermal loading as compared to conventional mask designs.